I suspect you are using Layer Styles to add a Stroke around your text. If that's true then you'll have to simplify the layer twice, once to change the text to an image and again to change the style too.

Then you can use the Magic Wand tool to select the stroke color then invert it to make it white.

It would be simpler though to simplify the text layer before adding the Stroke. Doing this, however, means it can no longer be edited as text.

Ensure that your foreground color is white
Cmd-click on the layer thumbnail to select the text
Click Edit > Stroke, and set the width and color of your outline before clicking OK.